JP-8-144758A shows an engine cooling system in which engine coolant is circulated in order to cool a cylinder head and cylinder block of an engine. A mechanical water pump circulating the engine coolant is driven by a driving force transmitted from a crankshaft of the engine. While the engine is running, the mechanical water pump is also driven in order to circulate the engine coolant. A combustion chamber of the engine is also cooled, so that anti-knocking ability is improved.
If the engine is shut down with high temperature, the cylinder head temperature may be greater than a specified temperature when the engine is restarted. This specified temperature is established for improving the anti-knocking ability. If the engine is restarted with the cylinder head of high temperature, fuel consumption efficiency may be deteriorated.
Especially, in a vehicle having an idle reduction function and a vehicle having a hybrid engine, the equipped engine is frequently stopped and restarted. Thus, the above problems often occur.
U.S. Pat. No. 5,337,704 shows an engine cooling system in which engine coolant passed through a cylinder-head passage flows into a heat exchanger for heating a passenger compartment.
EP-1008474A1 shows a heating system which includes two heat exchangers into which engine coolant is respectively introduced.
In order to improve the anti-knocking ability, a cylinder head should be positively cooled.
Meanwhile, in order to restrict an increase in friction in an engine, a cylinder block should be kept at specified temperature or more. A cylinder-head-passage and a cylinder-block-passage are formed in the system, and coolant flow rate flowing through the cylinder-head passage is made larger than that flowing through the cylinder-block-passage.
When the engine coolant passed through only the cylinder head is used as a heat source for heating a passenger compartment, it is likely that the air temperature can not be increased enough.